The present invention relates to paper machines.
In particular, the present invention relates to paper machines having twin-wire formers.
Thus, the present invention relates to a paper machine having a twin-wire former which is provided by way of a pair of endless wire loops which travel upwardly along a common path having a lower receiving end to receive pulp stock from which a web is to be formed and an upper discharge end from which a web issues while being transported beyond the common path by one of the wire loops. Lower and upper forming rolls cooperate with the endless wire loops respectively at the lower and upper end regions of their common path, and in advance of the lower forming roll the paper machine includes a breast roll as well as a headbox which supplies pulp stock to either one or both of the endless wire loops or into a throat defined therebetween at the receiving end of the common path.
Twin-wire formers are utilized in paper machines where the web forming and dewatering of the web while it is being formed takes place primarily or at least to a great extent between two wires. Efforts are always made to provide symmetrical dewatering by way of a twin-wire former, such symmetrical dewatering implying that the dewatering takes place in equal amounts through both of the wires. This result is achieved with known constructions by providing for simultaneous dewatering in opposite directions through both of the wires. Symmetry of the dewatering action is in itself no particular objective but it is worthwhile to achieve because of the fact that it has a great influence on the uniformity of the distribution of the material which forms the paper. Thus, for example, this factor will determine the uniformity of the distribution of fine fibers included in the fiber composition of the paper as well as the distribution of materials such as fillers which are distributed through the thickness of the paper.
Known twin-wire formers generally fall into two main categories. One category has the common run of the wires nearly straight or only slightly curved at the location where the major part of the dewatering occurs. This latter slight curvature may have a radius on the order of, for example, 3,000 mm or more. The joint or common path of travel of the wires is determined, for example, by a guiding action provided by way of at least one stationary shoe and/or several rotating guide rolls. This particular category also includes those known twin-wire formers where both wires have a substantially vertical orientation, so that they define between themselves a formation region which is of a wedge-shaped configuration.
The other main category of twin-wire former which is known in the art is a type of twin-wire former where dewatering of the web, while it is formed, takes place at a so-called forming roll an angular portion or sector of which is lapped by both wires. Such a forming roll will have a diameter on the order of approximately, for example, 1,000-2,000 mm, with the radius of curvature thereof correspondingly being on the order of 500-1,000 mm. The angular region of such a forming roll over which dewatering takes place is on the order of, for example, 90.degree.-120.degree.. These dimensions and ranges are understood as examples only, intended to illustrate the differences existing between the different categories. Such dimensions may depend, for example, on the breadth of the machine, and these dimensions are not intended to restrict the scope of the present invention in any way. The starting point of the present invention resides precisely in twin-wire formers of the above categories.
It is conventional to provide in twin-wire formers a forming roll which usually is a suction roll or another type of roll with a very open construction. However, suction rolls are exceedingly expensive components. The cost thereof increases sharply with an increase in diameter. Such suction rolls may be made of special alloys so that, even though they are perforated, nevertheless they will still have the required strength for a paper machine of great breadth and so that they will behave durably with respect to adverse corrosion conditions. Moreover, the drilling of holes through the shells of suction rolls involves high manufacturing costs.
Furthermore, the use of a suction roll requires also a connection to a vacuum system. In order to generate the vacuum a suction pump is required, so that in this latter connection also there are high energy costs.